1. Field of the Invention
The present invention relates to a control apparatus for controlling a wireless communication system, a communication apparatus, and a control method therefor.
2. Description of the Related Art
Recently, in office environments where networking is advancing with widespread use of the Internet, there are growing needs for replacing part of wired LANs (Local Area Networks) typified by the Ethernet (registered trademark) with wireless LANs that dispense with complicated wiring.
Further, a system has conventionally been proposed (e.g. in US Published Patent Publications No. 2002-0080739 and No. 2002-0080855) in which when a Bluetooth device and a wireless LAN use the same frequency range, the frequency range itself is divided so as to enable one of them to avoid the use of a frequency range being used by the other.
Further, a wireless LAN using an ISM (Industrial, Scientific Medical) band has already been standardized as the IEEE 802.11 standard, and the IEEE 802.11b standard adapted to increased transmission speed has also been put to practical use.
The MAC (Media Access Control) of the IEEE 802.11 standard uses a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol, which prescribes that, in transmitting data, detection of a carrier on a wireless transmission medium should be performed; if the carrier is detected, the data transmitting operation should be temporarily stopped (data transmission should be deferred); and after the carrier ceases to be detected, the data transmitting operation should be resumed upon the lapse of a randomly chosen transmission waiting time (random back-off time). Further, to reduce the collision probability of data, the MAC of the IEEE 802.11 standard (hereinafter referred to as “the IEEE 802.11 MAC”) uses the technique of Distributed Coordination Function (DCF) in which a data transmission request signal RTS (request to send) is transmitted in advance, and after receiving a transmission permission signal CTS (clear to send), transmission of a data packet is performed, and confirmation of the transmission of the data packet is carried out by receiving an acknowledgement (ACK). The IEEE 802.11 MAC thus solves a problem caused by contention or collision on the wireless transmission medium.
Furthermore, the IEEE 802.11 MAC defines the Point Coordination Function (PCF) as a means for securing a contention free period for an access point (AP) where access concentration of a plurality of wireless communication terminals can occur.
As shown in FIG. 11, the PCF secures a contention free period on a wireless medium by causing an access point to periodically transmit a beacon containing contention avoidance time information. A NAV (Network Allocation Vector) obtained, as a contention free period, in response to the beacon is set to a CFP max duration, whereby transmission from stations (STAs) for which polling is inhibited. The access point performs polling to wireless communication terminals on the network during the contention free period, to thereby license the terminals to carry out transmission and hence realize contention free transmission. The value of the NAV is updated whenever a beacon is transmitted.
A time interval between frames transmitted to the medium is defined as an IFS (Inter Frame Space), and a wireless communication terminal uses a carrier sense function in a prescribed IFS interval to determine whether the medium is idle. The PCF defines the time of a SIFS (Short Inter Frame Space) as a standby time for awaiting reception of a response to polling by the access point. When no response is obtained within the SIFS time period after transmission of a polling packet, the next polling packet is immediately transmitted.
The PCF defines a PIFS (PCF Inter Frame Space) set to a longer time period than the SIFS, and when the medium has been idle over this time period, the access point is allowed to send a CF-polling frame to a next station. Further, By setting the beacon repetition period to a longer time period than the contention free period, the access point can operate on both the PCF and the DCF. Furthermore, when it is determined that there is no data to be transmitted within a secured contention free period, a CF-END (Contention Free-END) signal is transmitted to terminate the contention free period, whereby a time period before transmission of the next beacon can be used as the DCF period.
In the IEEE 802.11 standard (hereinafter referred to as “the wireless LAN”), a plurality of frequency channels are defined, whereby a plurality of access points can be operated using the respective different frequency channels in an environment where the access points are independent of each other and free of mutual interference.
On the other hand, in the digital wireless communication technology, various methods have been put to practical use, and with a view to replacement of cables connecting between communication apparatuses and achievement of low costs and low power consumption, researches on Bluetooth devices using a simplified wireless communication method are being carried out.
In the Bluetooth technology, the communication procedure is determined as a profile depending on the kind of data to be handled, whereby wireless connection between various kinds of communication apparatuses or devices is achieved. For example, in the case of establishing connection to a main LAN within an office, the connection can be achieved via an access point using a LAN or PAN (Personal Area Network) profile, while in the case of establishing connection to the main LAN from outside the office, the connection can be achieved via a cellular phone with the Bluetooth technology, using a DUN (Dial Up Network) profile.
As is apparent from the above fact, the Bluetooth technology is expected to be widely used in portable information apparatuses and devices, such as cellular phones, notebook PCs (Personal Computers), and PDAs (Personal Digital Assistants), for business use. The Bluetooth technology also uses an ISM band, similarly to the wireless LAN.
The Bluetooth network topology is a star network of centralized control type, and communication apparatuses and devices which are linked with each other include a single control station, referred to as the master, which controls data traffic, and slaves which perform data transfer under the control of the master.
To establish a link, each slave synchronizes the frequency of a communication channel and transmission/reception timing using a hopping pattern generated from a BD_ADDR (Bluetooth Device Address) of the master, in clock timing synchronized with a system clock (native clock) of the master.
The BD_ADDR is uniquely assigned to each communication apparatus or device, so that a plurality of communication apparatuses and devices can use the same band and perform communication using different hopping patterns, which makes it possible to operate Bluetooth devices independently as in the case of wireless LAN devices.
In the MAC defined by the Bluetooth technology, up to a maximum of seven slaves can be connected to a single master, and as shown in FIG. 12, the master sequentially executes polling control of the linked slaves to thereby give each slave a license to transmit.
When the master has data to send to a specific slave, a polling signal from the master is sent as a data packet to the slave, whereas when the master has no data to send to the slave, the polling signal from the master is sent as a polling packet to the slave. When receiving one of the packets, the slave performs transmission to the master using a time slot immediately after completion of the packet reception.
The transmission of one packet can extend up to a maximum of five time slots in one direction. This extension is possible for a packet transmitted from the master to a slave or from a slave to the master, or for packets transmitted in both the directions, and assignment of the extended slots is finally determined by a link manager protocol of the master.
In addition to the packet data transmission using the so-called ACL (Asynchronous Connectionless) link described above, the Bluetooth technology has realized voice communications using an SCO (Synchronous Connection Oriented) link that reserves communication slots to be used. When the SCO link is used, the master executes polling control as in the case of using the ACL link, but in the SCO link, the polling period is fixed so as to ensure real-time communications.
Further, the Bluetooth technology defines a power saving mode for temporarily stopping communicating operation when there is no data to be transferred after link establishment. In the power saving mode, a master suspends polling control of a slave having entered the power saving mode.
On the other hand, the slave saves electric power by stopping transmission/reception operation over a predetermined time period.
In the above-described background, these two different wireless communication techniques are both expected to be employed in portable information devices and apparatuses, such as notebook PCs. However, the two wireless communication methods are not compatible with each other, and therefore when Bluetooth-based communication is carried out during execution of wireless LAN-based communication for example, a Bluetooth signal and a wireless LAN signal can overlap each other, which causes interference.
At present, both the IEEE and the Bluetooth SIG (Special Interest Group) are studying the method of avoiding this interference. For example, a method is under study in which the states of wireless LAN-based communication and Bluetooth-based communication are monitored on the respective MAC levels of the wireless LAN and the Bluetooth, and one of the two communications is suspended during execution of the other.
However, the above-described conventional interference avoiding method is studied on the premise that each portable information device or apparatus such as a note PC is equipped with communication interfaces of the two different kinds, and therefore the method does not suffice to provide desired characteristics for an access point implementing both of the two different methods. More specifically, the Bluetooth technology allows up to a maximum of only seven slaves to be connected to a single master, and therefore an office or the like where communication terminals concentrate necessitates provision of a plurality of Bluetooth interfaces. On the other hand, if a wireless LAN is in an environment where a large amount of traffic occurs, it can be envisaged that the wireless LAN is equipped with a plurality of wireless LAN interfaces for decentralization of the traffic.
In the above-described conventional interference avoiding method, the two different kinds of wireless communication methods can be used together by controlling transmission/reception timing for each of the two kinds of wireless communication methods. However, this method does not contemplate provision of a plurality of wireless LAN interfaces and a plurality of Bluetooth interfaces, and therefore is incapable of controlling communication timing between interfaces of the same communication method or between interfaces of the different communication methods, which makes it difficult to avoid interference between communications according to the different wireless communication methods.